1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device that has a semiconductor element, typically a thin film transistor (TFT), that has a semiconductor film crystallized on an insulating surface.
2. Description of the Related Art
In electronic devices such as a television, a personal computer, and a cellular phone, a display for displaying characters or images is indispensable as a means for recognizing information. In these days especially, a flat-type semiconductor display device (flat panel display), represented by a liquid crystal display device that utilizes the electro-optical character of liquid crystal, has actively been used.
An active matrix driving method is known as one mode in the flat panel display. In the active matrix driving method, a TFT is provided in each pixel and image signals are in order written to perform a display of an image. The TFT is an essential element for realizing the active matrix driving method.
Although most TFTs were manufactured of an amorphous semiconductor films, electric field effect mobility (hereinafter, referred to a mobility) was low and it was impossible to operate the TFTs in accordance with frequency necessary for processing image signals. Therefore, the TFTs were used only as a switching element provided in each pixel. A gate line driving circuit for selecting pixels and a source line driving circuit for inputting image signals into the selected pixels were externally provided with IC (driver IC) prepared by TAB (Tape Automated Bonding) and COG (Chip on Glass).
As pixel density is higher, however, pixel pitch is narrower. Therefore, it is considered that the method of providing the drive IC has limit. For example, in the case of UXGA (pixel number: 1200×1600) supposed, 6000 connection terminals are necessary in RGB color method if an estimate is simply made. The increase of the number of connection terminals causes contact defects to increase and the occurrence probability of the contact defects increase as a result. Further, since a region of a periphery portion outside a pixel portion (frame region) increases, it is difficult to miniaturize a semiconductor device that has the above frame region and the design of appearances of the semiconductor device is spoiled.
Considering the above background, it is obvious that a semiconductor device provided with driving circuit in a body is necessary. When a pixel portion, a gate line driving circuit, and a source line driving circuit are formed together on the same substrate, in short, system-on-panel is realized, the number of connection terminals is sharply decreased and it is possible to reduce an area of the frame region.
As a means of realizing the system-on-panel, a method of forming a TFT of a polycrystal semiconductor film is proposed. Since the TFT of the polycrystal semiconductor film (polycrystal TFT) has higher mobility compared to a TFT formed of an amorphous semiconductor film, it is possible to operate the polycrystal TFT in accordance with frequency necessary for handling image signals. Therefore, when the polycrystal TFT is used, it is possible to realize a semiconductor device provided with a diver circuit, which has a pixel portion, a gate line driving circuit, and a source line driving circuit formed on the same substrate in a body.